Recent demands for multi-terabit communication require external amplitude modulators operating at low voltages. Amplitude modulators based on the Quantum Confined Stark Effect (QCSE) in Group III-V semiconductors multiple quantum well (MQW) systems are suitable for meeting these technological demands and, therefore much attention has been devoted to their development. One of the crucial requirements for efficient modulation at high bit rates is that the change in absorption per applied voltage is as large as possible. In other words, the Stark shift should be maximized. The larger the quantum well the larger the Stark shift. However, increasing the quantum well width decreases the oscillator strength for absorption. Thus, a compromise is imposed.
An alternative for increasing the Stark shift has been proposed by Batty and Allsop [1]. They have theoretically shown that the introduction of a so called nipi-δ-doping superlattice in a MQW structure where the quantum well is n-δ doped while the barrier is p-δ doped, may double the Stark shift.
δ-doping is a doping technique that attempts to spatially confine the dopant impurities to one or a few atomic layers during epitaxial growth of semiconductors. The basic idea of δ-doping superlattices is to create a periodic band edge modulation in real space without changes in the chemical composition of the semiconductor. There are several possible layer schemes which create a band edge modulation. One example is the nipi-δ-doping superlattice, which consists of a series of alternating n-type and p-type δ-doping layers separated by intrinsic layers. This type of superlattice causes a saw tooth shaped potential profile in the MQW-structure. The δ-doping layers are separated by the same distance, where, if the barrier between the doping planes is small enough, interaction is possible between the electronic wave functions of the neighboring δ-doping layers.
If the nipi-MQW structures are to be used in amplitude modulators, some requirements should be fulfilled. The presence of the nipi-δ-doping superlattice should not introduce energy levels in the forbidden gap, otherwise, in the ON state of the device, light could be absorbed, thereby dramatically increasing insertion losses. For applications where the MQW-structure forms the active region of the device, it is crucial that the net doping corresponds to an undoped structure, so that the applied electric field is uniformly distributed over the entire MQW region. It is therefore essential to balance out the electron and the hole concentrations in the δ-layers. However, the required balance between n-type doping levels and p-type doping levels is not trivial to achieve due to the presence of interface hole traps whose population depends on the quantum well doping concentration.
Prior art semiconductor amplitude modulators are adapted for 2.5 or 10 GHz. In order to reach higher frequencies (use lower ac voltages) it is fundamental that the Stark shift, and consequently the change in absorption, is larger.